1 /* $NetBSD: difftime.c,v 1.5 1997/07/13 20:26:48 christos Exp $ */ 2 3 /* 4 ** This file is in the public domain, so clarified as of 5 ** June 5, 1996 by Arthur David Olson (arthur_david_olson@nih.gov). 6 */ 7 8 #include <sys/cdefs.h> 9 #ifndef lint 10 #ifndef NOID 11 #if 0 12 static char elsieid[] = "@(#)difftime.c 7.7"; 13 #else 14 __RCSID("$NetBSD: difftime.c,v 1.5 1997/07/13 20:26:48 christos Exp $"); 15 #endif 16 #endif /* !defined NOID */ 17 #endif /* !defined lint */ 18 19 /*LINTLIBRARY*/ 20 21 #include "private.h" 22 23 /* 24 ** Algorithm courtesy Paul Eggert (eggert@twinsun.com). 25 */ 26 27 #ifdef HAVE_LONG_DOUBLE 28 #define long_double long double 29 #endif /* defined HAVE_LONG_DOUBLE */ 30 #ifndef HAVE_LONG_DOUBLE 31 #define long_double double 32 #endif /* !defined HAVE_LONG_DOUBLE */ 33 34 double 35 difftime(time1, time0) 36 const time_t time1; 37 const time_t time0; 38 { 39 time_t delta; 40 time_t hibit; 41 42 if (sizeof(time_t) < sizeof(double)) 43 return (double) time1 - (double) time0; 44 if (sizeof(time_t) < sizeof(long_double)) 45 return (long_double) time1 - (long_double) time0; 46 if (time1 < time0) 47 return -difftime(time0, time1); 48 /* 49 ** As much as possible, avoid loss of precision 50 ** by computing the difference before converting to double. 51 */ 52 delta = time1 - time0; 53 if (delta >= 0) 54 return delta; 55 /* 56 ** Repair delta overflow. 57 */ 58 hibit = (~ (time_t) 0) << (TYPE_BIT(time_t) - 1); 59 /* 60 ** The following expression rounds twice, which means 61 ** the result may not be the closest to the true answer. 62 ** For example, suppose time_t is 64-bit signed int, 63 ** long_double is IEEE 754 double with default rounding, 64 ** time1 = 9223372036854775807 and time0 = -1536. 65 ** Then the true difference is 9223372036854777343, 66 ** which rounds to 9223372036854777856 67 ** with a total error of 513. 68 ** But delta overflows to -9223372036854774273, 69 ** which rounds to -9223372036854774784, and correcting 70 ** this by subtracting 2 * (long_double) hibit 71 ** (i.e. by adding 2**64 = 18446744073709551616) 72 ** yields 9223372036854776832, which 73 ** rounds to 9223372036854775808 74 ** with a total error of 1535 instead. 75 ** This problem occurs only with very large differences. 76 ** It's too painful to fix this portably. 77 ** We are not alone in this problem; 78 ** some C compilers round twice when converting 79 ** large unsigned types to small floating types, 80 ** so if time_t is unsigned the "return delta" above 81 ** has the same double-rounding problem with those compilers. 82 */ 83 return delta - 2 * (long_double) hibit; 84 } 85